Waste gas flare assemblies are commonly located at production facilities, refineries, processing plants and the like (collectively “facilities”) for disposing of flammable gas streams that are released due to venting requirements, shut-downs, upsets and/or emergencies. Such flare assemblies are typically required to accommodate waste gases that vary in composition over a wide range and operate over a very large turndown ratio (from maximum emergency flow to a purge flow rate) and extended periods of time without maintenance.
A typical single-point flare assembly includes a flare riser, which can extend a few feet to several hundred feet above the ground, and a flare tip mounted to (e.g., in a vertical flare, on the top of) the flare riser. The flare tip typically includes one or more pilots for igniting the vent gas. Depending on the particular flare tip design and available gas pressure, some flares include smoke suppression equipment such as steam injectors or air blowers.
Waste gas can be released at any time during operation of a facility. As a result, an integrated ignition system that can immediately initiate burning throughout the period of waste gas flow is critical. An integrated ignition system includes at least one pilot, at least one pilot ignition mechanism and at least one pilot flame monitor. Pilot gas must generally be supplied to the flare pilot at all times.
Due to various process and/or regulatory considerations, various other gases are sometimes added to the released waste gas stream. Examples of other gases that are sometimes added to the released waste gas stream include purge gas (for example, natural gas or nitrogen) and enrichment fuel gas (for example, natural gas or propane). The gas stream that arrives at the inlet of the flare tip is referred to as “vent gas,” regardless of whether it consists of only the released waste gas or the released waste gas together with other gases that have been added thereto. The vent gas together with all other gases and vapors present in the atmosphere immediately downstream of the flare tip, not including air but including steam added at the flare tip and fuel gas discharged from the pilot(s) of the flare assembly, is referred to as “flare gas.”
Purge gas is often added to the released waste gas stream (or otherwise to the flare assembly if a waste gas stream is not being released by the facility at the time) in order to maintain a positive gas flow through the flare assembly and prevent air and possibly other gases from back flowing therein. Enrichment fuel gas is sometimes added to the waste gas stream to help assure that the required minimum net heating value of the vent gas is met. Current regulations in the United States relating to flares (such as the regulations at 40 C.F.R. §60.18) specify that the net heating value of the vent gas is to be no less than 300 British thermal units (Btu's) per standard cubic foot (scf). Certain consent decrees between flare owners and the U.S. Environmental Protection Agency (the “EPA”) may specify that the net heating value of the vent gas must be even higher than 300 Btu/scf. Whether an enrichment fuel is used, as well as the amount of enrichment fuel used, will depend on the composition of the waste gas stream, the flow rate of the waste gas stream and applicable regulations relating to operation of the flare.
Most gas flares are required to operate in a relatively smokeless manner. This is achieved by making sure that the vent gas is admixed with a sufficient amount of air in a relatively short period of time to sufficiently oxidize the soot particles formed in the flame. In applications where the gas pressure is low, the momentum of the vent gas stream alone may not be sufficient to provide smokeless operation. In such applications, it is necessary to add an assist medium to achieve smokeless operation. The assist medium can be used to provide the necessary motive force to entrain ambient air from around the flare apparatus. Examples of useful assist media include steam and air. Many factors, including local energy costs and availability, must be taken into account in selecting a smoke suppressing medium.
The most common assist medium for adding momentum to low-pressure gases is steam, which is typically injected through one or more groups of nozzles that are associated with the flare tip. In addition to adding momentum and entraining air, steam also dilutes the gas and participates in the chemical reactions involved in the combustion process, both of which assist with smoke suppression. In one simple steam assist system, several steam injectors extend from a steam manifold or ring that is mounted near the exit of the flare tip. The steam injectors direct jets of steam into the combustion zone adjacent the flare tip. One or more valves (which can be remotely controlled or automatically controlled) adjust steam flow to the flare tip. The steam jets inspirate air from the surrounding atmosphere and inject it into the discharged vent gas with high levels of turbulence. These jets may also act to gather, contain, and guide the gases exiting the flare tip. This prevents wind from causing flame pull down around the flare tip. Injected steam, educted air, and the vent gas combine to form a mixture that helps the vent gas burn without visible smoke. Other steam assist systems have been developed and successfully utilized in connection with more complex flare systems.
Most steam-assisted flares require a minimum steam flow in order to keep the steam line from the control valve to the flare tip warm and ready for use and to minimize problems with condensate in the steam line. Also, a minimum steam flow keeps the manifold and other steam injection parts on or near the flare tip cool which helps prevent heat damage thereto (for example, in the event a low flow flame attaches to the steam equipment).
Operation of a flare assembly in freezing conditions creates additional issues that must be addressed. For example, when steam is discharged through the flare assembly at a low flow rate to cool the steam equipment when the flare is in a standby condition or to assist a low volume flaring event, freezing temperatures may cause the steam to condense and form ice on or around the flare tip. Also, condensation can occur in the steam line running from the source of steam to the flare assembly. In some cases, the steam line is very long and, despite the use of insulation, prone to condensation. The condensation can be sprayed at the flare tip and ultimately freeze in or around the flare tip and associated equipment. The formation of ice on or around the vent gas discharge opening, for example, can lead to blockage of the discharge opening and other serious problems.
As the flow rate and/or composition of vent gas sent to a flare tip varies, the amount of steam required for smoke suppression changes. Many plants adjust the steam requirement based on periodic observations by an operator in the control room looking at a video image from a camera monitoring the flare. Smoking conditions may be corrected by increasing the rate of steam flow to the flare. However, when the vent gas flow begins to subside, the flare flame may continue to look “clean” to the operator, which may allow some time to pass before the operator reduces the steam flow. As a result, this method of smoke control tends to result in over-steaming of the flare which in turn may lead to excessive noise and unnecessary steam consumption, low destruction and removal efficiency, or even extinguish the main flame altogether.
Too much steam can cause the ratio of the flow rate of steam discharged by the flare assembly to the flow rate of vent gas discharged by the flare assembly (the “steam/vent gas ratio”) to become too high, which can in turn reduce the net heating value of the flare gas in the combustion zone to a point that combustion cannot be sustained. This can particularly be a problem when the vent gas flow rate is at a low level. It can also be a problem when the flare assembly is in standby condition, and there is only minimum flow of purge gas through the stack. Allowing the steam/vent gas ratio to exceed a certain level and the net heating value of the flare gas to become too low may violate one or more regulations relating to operation of the flare assembly.
A wide variety of factors impact the destructive removal efficiency (DRE) of a flare, including ambient conditions, vent gas flow rate and composition, vent gas exit velocity, steam flow rate, steam exit velocity, the amount of air entrained by the steam, how well and how rapidly the steam and entrained air mix with the vent gas, and the design of the flare tip. As a result, it is difficult to specify simple operating parameters that ensure a high DRE and prevent over-steaming.
Flare vendors typically require a minimum standby steam flow rate for purposes such as keeping the steam line warm and preventing the steam injector assembly and related equipment from heat damage. The flow rate of the steam cannot be reduced below the minimum standby rate recommended by the flare vendor without risking problems such as the problems described above. Furthermore, a lower rate of steam may not be sufficient to achieve smokeless operation, which may also violate applicable regulations regarding visible emissions and is undesirable in most applications. Due to the low exit velocity and resulting low air entrainment rate of steam at turndown steam rates, it takes a higher steam/vent gas ratio to achieve smokeless operation of a flare than that required when steam is injected at sonic velocity. Under some circumstances, both smoking and over-steaming, as legally defined by applicable regulations, cannot be avoided at the same time in a conventional steam assisted flare, no matter how the steam flow rate is adjusted. Increasing the purge gas flow rate (as opposed to reducing the steam flow rate) may help with compliance but the costs of the increased purge gas may be prohibitive. The increased purge gas may also contribute to higher emissions of carbon dioxide, a gas related to greenhouse effects. This can create a dilemma for owners of steam-assisted flares with respect to operation of the flare.
A primary purpose of a flare assembly is to destroy and control potentially harmful compounds such as sulfur compounds, carbon monoxide and unburned hydrocarbons. As a result, the operation of a flare assembly is regulated and monitored by various governmental agencies. The particular regulations that apply depend on the particular location of the flare assembly. In the United States, for example, the operation of a flare assembly is regulated and monitored by the EPA. Flare regulations in the United States include regulations in the Code of Federal Regulations (CFR) and settlement agreements (for example, consent decrees) reached between regulating agencies such as the EPA and facilities. State and local regulations may also apply.
It is anticipated that more stringent regulations with respect to operation of a flare assembly may be implemented by the EPA in the near future. These new regulations may be in the form of consent decrees reached between the EPA and flare owners, or may be made a part of the applicable Code of Federal Regulations. The new regulations will likely address, for example, the maximum steam/vent gas ratio (or steam/hydrocarbon ratio) that can be employed, the minimum net heating value of the vent gas, and the minimum net heating value of the flare gas in the combustion zone. In view of these regulations, it may become even more difficult for a conventional steam-assisted flare assembly to achieve smokeless operation, prevent over-steaming and address other problems such those described above. Simply reducing the amount of steam may not be a sufficient solution.